Condensates from the digester or the evaporation areas of a kraft pulp mill are contaminated with volatile organics, sulfur-containing components, fibers, and black liquor carryover. More than 60 different compounds have been detected in foul evaporator condensate from a kraft pulp mill. The major pollutants of concern are total reduced sulfur (TRS) compounds and methanol. The main TRS components are: hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3) and dimethyl disulphide (CH3SSCH3). Other organic compounds found in digester and evaporator condensates include: ethanol, acetone, methyl ethyl ketone, terpenes, phenolics, and resin acids. A typical kraft mill produces about 7 to 15 kg of methanol per ton of pulp.
Due to the presence of TRS compounds, the foul condensate cannot be used within the mill. Mills which sewer a portion of their condensates must increasingly deal with several problems such as: additional BOD loading to the effluent treatment system, emission of odorous compounds and cooling of the condensates before discharge. Several treatments such as biological treatment, thermal oxidation and chemical oxidation have been devised to remove TRS and methanol from this stream. U.S. Pat. No. 6,579,506, for example, dealt with the chemical oxidation of a gas stream containing the above TRS compounds using chlorine dioxide solution.
Steam stripping technology has been the predominant choice for most mills for the treatment of foul condensate. In a steam stripper, the foul condensate is fed close to the centre of the stripping column after being heated by the clean condensate exiting the stripper. Steam is fed into the bottom of the column. The upper part of the column acts as a rectifier that separates and concentrates the condensable from the non-condensable gases. Stripping takes place in the bottom section of the column. The stripped gases are cooled in a reflux condenser and the condensate is collected in a reflux tank. The stripper-off gases are separated and sent to be burnt in a kiln, a boiler or an incinerator. The liquid portion is fed back to the column from the top. Incineration is an expensive approach for the disposal of methanol and TRS-rich gaseous streams since it requires a considerable amount of energy and a scrubber for the generated SO2. In addition, methanol, a valuable chemical, is destroyed.
At several kraft pulp mills, purchased methanol is used in the chlorine dioxide generator as a reducing agent for the production of chlorine dioxide.
In such generators, sodium chlorate, sulphuric acid and methanol are mixed together and reacted under certain well-defined conditions to yield ClO2. There exist several patents on the production of ClO2 using pure methanol under various operating conditions. These include: U.S. Pat. No. 5,676,920, U.S. Pat. No. 5,066,477, U.S. Pat. No. 4,473,540, U.S. Pat. No. 4,465,658 and U.S. Pat. No. 4,145,401.
As mentioned before, the steam stripper-off gases are rich in methanol and TRS compounds. No value-added uses of this stream have been suggested or investigated in the prior art. Only destruction approaches have been considered to address the odour and toxicity issues associated with the TRS components.
Similar problems arise with condensates from other classes of chemical pulp mill, for example sulphite pulp mills.